Rhizosphere size and shape: Temporal dynamics and spatial stationarity
The soil volume affected by roots – the rhizosphere – is one of the most important microbial hotspots determining the processes, dynamics and cycling of carbon (C), nutrients and water in terrestrial ecosystems. Rhizosphere visualization is necessary to understand, localize and quantify the ongoing processes and functions, but quantitative conclusions are very uncertain because of: 1) the continuum of the parameters between the root surface and root-free soil, i.e., there are no sharp borders, 2) differences in the distributions of various parameters (C, nutrients, pH, enzyme and microbial activities, gases, water etc.) across and along roots, 3) temporal changes of the parameters and processes with root growth as well as with water and C flows. In situ techniques: planar optodes, zymography, sensitive gels, ¹⁴C and neutron imaging as well as destructive approaches (thin layer slicing) have been used to analyze the rhizosphere extent and the gradients of various physico-chemical and biological characteristics: pH, CO2, O2, redox potential, enzyme activities, content of water, nutrients and excess elements, and organic compounds. A literature analysis allows the conclusion that: i) the rhizosphere extent for most of the parameters assessed by non-destructive visualization techniques is 0.5–4 mm but exceeds 4 mm for gases, nitrate, water and redox potential. ii) The rhizosphere extent of nutrients (N, P) is decoupled from the extent of the corresponding enzyme activities. iii) The imbalance between element flows to and uptake by roots may lead to accumulation of excess elements and formation of root carapaces (e.g. CaCO3 rhizoliths, Fe plaque) ranging up to a few cm. iv) All destructive approaches show a much (3–5 times) larger rhizosphere extent compared to visualization techniques. These conclusions are crucial for a mechanistic understanding of rhizosphere properties and functioning, estimation of the nutrient stocks available to roots, and for rhizosphere modelling considering root growth and architecture. Overall, roots function as ecosystem engineers and build their environment, serving as the main factors shaping rhizosphere extent. Sharp gradients are formed within a few days for nutrients and enzymes, but more time is necessary for the establishment of specific microbial communities. Despite the very strong dynamics of most parameters, their stationarity is reached within a few days because the release of C and enzymes or nutrient uptake are very quickly compensated by utilization by surrounding microorganisms or/and sorption and diffusion processes. We conclude that despite the dynamic nature of each property, the rhizosphere gradients, their extent and shape are quasi-stationary because of the opposite directions of their formation processes.